A number of photovoltaic solar concentration (PSC) systems have been proposed and developed throughout the twentieth century until present day. Despite this long history, these state of the art systems are not currently competitive in terms of cost and efficiency with respect to the traditional forms of producing energy.
Documents WO2006114457, US2009106648 and WO2009058603 show the typical operating scheme of a photovoltaic solar concentration system. Said system consists of a light concentrating Fresnel lens and a secondary optical element providing the system with greater concentration. Various systems using Fresnel lenses have been proposed with and without secondary optics.
Concentration systems by means of light guiding have recently been reported, as described in document WO2008131566. Said systems are characterized by their greater compactness compared to traditional systems.
There are other photovoltaic solar concentration systems based on Cassegrain technology. Said systems consist of a pair of reflecting mirrors and a homogenizing tertiary optical element. There are also other optical concentration elements based on parabolic mirrors. Said systems can be formed by mirrors or they can be an entirely solid system based on Complete Internal Reflection (CIR), as described in documents WO2009058603 and WO2009086293.
Different types of tertiary optical elements, such as prisms with straight or curved walls, or Kohler type systems, as can be observed in FIG. 2, in which tertiary optical elements are not used, have been disclosed for said Cassegrain systems.
An ideal photovoltaic solar concentration system must have the following features in order to be competitive: it must minimize losses in optical concentration systems, i.e., achieve greater optical efficiency; they must be cost effective solutions with long-term reliability; they must be compact and achieve maximum thermodynamic efficiency, i.e., they must reach the maximum degree of concentration possible in a design that absorbs the manufacturing tolerances of the system and deviations of the solar tracker.
Maximizing thermodynamic efficiency means maximizing the use of the etendue. The concept of etendue was described by Dr. Winston et al. in Non Imaging Optics and is highly important in a photovoltaic solar concentration system. Maximizing the etendue means maximizing the acceptance angle of a system for a particular degree of concentration, or maximizing the concentration for a defined acceptance angle. A maximum etendue use module has the potential to effectively concentrate solar radiation, minimizing the cost of the semiconductor element and accordingly of the module. It also provides the system with the tolerance necessary for being assembled in real solar tracking systems and allowing the manufacturing tolerances of the module without it affecting the output thereof.
The maximum degree of concentration that can be attained for an acceptance angle is defined by the following equation:
      C    ⁢                  ⁢    max    =            (                        n          2                ·                              sine            ⁡                          (                              θ                1                            )                                2                    )              (              sine        ⁢                              (                          θ              2                        )                    2                    )      wherein n is the refractive index of the medium in which is submerged the photovoltaic receiver, θ1 is the angle of entry in the photovoltaic cell and θ2 the acceptance angle in the system. Maximizing the use of the etendue involves being as close as possible to Cmax for defined n, θ1 and θ2.
The photovoltaic solar concentration systems by means of Fresnel lenses have been the most widely used up until now because it is a known, standard and cost effective technology. However, they are not excessively compact systems and they do not maximize the etendue use. Nevertheless, certain documents have been published for the purpose of maximizing the etendue use using lens systems with very high focal distances and secondary elements with a certain curvature at the entry.
Reflective systems are progressively being introduced, are generally more compact than refractive systems, and with the suitable design, maximize etendue use compared to lenses.
The light guiding systems are the most compact. However they have yet to prove their optical efficiency, costs and long-term reliability.
A system achieving a high photovoltaic solar concentration that prevents the drawbacks existing in the earlier systems in the state of the art was therefore desirable.